In general, a hydraulic system includes one or more hydraulically-actuated devices connected to a source of pressurized fluid. One example of such a system includes the hydraulically-actuated fuel injection systems manufactured by Caterpillar, Inc. of Peoria, Ill. for use on diesel engines. In current systems of this type, a plurality of hydraulically-actuated fuel injectors are mounted in an engine and connected to a common rail containing high pressure lubricating oil. The common rail is maintained pressurized by a fixed displacement pump that is driven directly by the engine. The pressure in the common rail is controlled by a conventional electronic control module that maintains pressure at a desired level by continuously dumping an amount of the pressurized oil back to the sump. While these hydraulically-actuated fuel injection systems have performed magnificently for many years, there remains room for improvement. In particular, controlling fluid pressure by dumping a portion of the pressurized fluid back to the oil pressure sump amounts to a waste of energy, which reveals itself as a higher than necessary brake specific fuel consumption for the engine. Thus, there remains room for improvement in the overall efficiency of the hydraulic system and engine if pressure in the common rail can be maintained and controlled without an excessive waste of energy through dumping pressurized fluid back to the sump.
The present invention is directed to these and other problems associated with pumps for hydraulic systems.